Give the formulas for wave number and time period.

$A$ hypothetical electromagnetic wave is shown below. The frequency of the wave is $x \times 10^{19} \ Hz$. $x =$ . . . . (nearest integer)

Calculate the energy per mole of photons of electromagnetic radiation having a wavelength of $700 \ nm$. $\left[h = 6.626 \times 10^{-34} \ J \ s, c = 3 \times 10^8 \ m/s, N_A = 6.022 \times 10^{23} \ mol^{-1}\right]$

Choose the correct statements in reference to the photoelectric effect. $(i)$ There is no time lag between the striking of light and ejection of electrons from the metal surface. $(ii)$ The number of electrons ejected is independent of the intensity of light. $(iii)$ The elements $K$, $Rb$ and $Cs$ can show photoelectric effect when exposed to the beam of light.

The $Cs^+$ ion imparts a violet color to the flame. How is this related to the emitted radiation?

Calculate the energy of one mole of photons of radiation whose frequency is $5 \times 10^{14} \ Hz$.